Rheology of polymer flocculated digested sludge: Insights from Couette and Poiseuille Flow

In this study, the rheological behaviour of polymer flocculated digested sludge was characterized over a range of solids concentrations (2.6 – 6.3 wt%) using both Poiseuille rheometry (pipes with internal diameters of 47.6, 60.3, 73 and 98.4 mm in a pipe loop and Couette rheometry with small and large gap geometries. Large-gap Couette measurements showed good agreement with Poiseuille flow behaviour, whereas small-gap measurements frequently overestimated viscosity due to stronger interactions between large polymer-bridged flocs and the confined gap. Comparison of unsheared grab samples from the thickener outlet with highly sheared pipe-loop samples revealed substantial differences in viscosity, demonstrating that shear degradation and slight solids-concentration variations can significantly influence rheological behaviour.